CN1293933C - Process for reducing NOx in waste gas streams using chlorine dioxide - Google Patents

Abstract

A method for reducing the _NOx concentration in an exhaust gas stream. More specifically, the present invention relates to contacting an NOx _- containing exhaust gas stream with an effective amount of chlorine dioxide under conditions in which at least a portion of oxidizable NOx _- like substances present in the exhaust gas stream are oxidized to oxides of nitrogen in higher oxidation states.

Description

Method for Reducing NOx in Exhaust Streams Using Chlorine Dioxide

technical field

The present invention relates to a method for reducing the _NOx concentration in an exhaust gas stream. More particularly, the present invention relates to combining an exhaust stream _{containing NOx} with an effective amount of di Chlorine oxide exposure.

Background technique

Increasingly stringent government regulatory emission standards have prompted refiners to develop improved technologies to reduce the concentration of oxides of nitrogen ("NO _x ") in combustion emissions and production effluent or waste streams. For example, the technique set forth in U.S. Patent No. 3,957,949 to Senjo et al. (which is hereby incorporated by reference) describes a method to release The oxidant removes low-soluble pollutants (such as mercury and NO) from exhaust gas streams. In addition, U.S. Patent No. 6,294,139 to Vicard et al. (which is also incorporated herein by reference) also discloses a process by oxidizing nitrogen oxides with chlorine dioxide or ozone and then allowing the oxidized gas to dissolve in an aqueous solution. A process for removing nitrogen oxides from an exhaust stream by contacting with sodium chlorite. Additionally, reducing _NOx concentrations in combustion exhaust streams by injecting ammonia is known in the art, see US Patent No. 3,900,554 to Lyon, which is also incorporated herein by reference. After Lyon's patent, patents and publications involving the injection of ammonia into the combustion stream to reduce _NOx concentrations skyrocketed. Such patents include US Patent Nos. 4,507,269 and 4,115,515, both of which are incorporated herein by reference.

Nonetheless, emissions from combustion units and production streams, such as regenerator off-gases of fluid catalytic cracking ("FCC") units, remain sources of _NOx emissions from refineries. Many FCC process units incorporate wet gas scrubbers for the removal of attrition catalyst fines. Wet gas scrubbers play a supporting role in reducing _NO2 emissions. Although scrubbing is effective in reducing NO _emissions , it is not effective in reducing NO emissions. Since the majority of the _NOx (typically 90%) contained in the exhaust stream of an FCC unit is NO, there is a need for a method for reducing NO emissions in the exhaust gas (or "tail gas") of an FCC unit so that Further reduce overall _NOx emissions.

One method of reducing NO emissions involves the oxidation of lower valence oxide _NOx species to higher valence nitrogen oxides. However, traditional methods either involve chemicals that require prolonged reaction times, or use chemicals that create problems in the operating setup. These problems include, for example, corrosion of materials of construction, disposal problems of wastewater from the plant, and problems related to removal of often co-existing _SOx species. For example, it is known in the art to add sodium chlorite ( _NaClO2 ) to wet gas scrubber fluids in order to oxidize _NOx species to higher valence oxides, such as _NO2 and _{N2O 5} . NO ₂ and N ₂ O ₅ are water soluble and can usually be removed from treatment systems in the form of nitrate and nitrite, respectively.

However, the method of adding sodium chlorite to the wet gas scrubber fluid has some disadvantages. For example, sodium chlorite is an expensive chemical and is consumed by side reactions such as the oxidation of SO _x species to higher valence sulfur oxides (eg, SO ₂ to SO ₃ ). Therefore, traditional methods require relatively high levels of chlorine dioxide in the wet gas scrubber fluid since sodium chlorite cannot selectively oxidize the lower valence oxides _NOx to higher valence nitrogen oxides. Sodium acid concentration can be reduced to reduce oxidizable NO _x species as desired. High concentrations of sodium chlorite produce high concentrations of chloride, which primarily cause corrosion of the materials of construction of the moisture scrubber.

Therefore, there remains a need in the art for a cost-effective method of reducing the amount of _NOx species in an exhaust gas stream.

Summary of the invention

According to the present invention, there is provided a method for reducing the concentration of _NOx in an exhaust gas stream containing both NOx and _SOx compounds, _the method comprising:

a) removing at least a portion of _SOx species from said exhaust stream, thereby producing a SOx _- depleted exhaust stream;

b) contacting said _SOx- depleted exhaust stream with an effective amount of chlorine dioxide under oxidizing conditions effective to oxidize at least a portion of the oxidizable _NOx species to higher valence nitrogen oxides; and

c) removing at least part of said higher valence nitrogen oxides from the treated exhaust stream by a method selected from the group consisting of alkaline solution absorption, reducing solution absorption, water washing, ammonia injection and catalytic conversion.

In a preferred embodiment, the chlorine dioxide is contacted with the waste gas stream using the nozzles necessary for the separation drum of the wet gas scrubber.

In another preferred embodiment, at least a portion of the NO _x species initially present in the exhaust gas stream is removed prior to step a) above.

Detailed description of the invention

As used herein, the terms " _NOx ", " _NOx " species, and "nitrogen oxides" refer to various oxides of nitrogen that may be present in combustion exhaust gases. Thus, these terms refer to all the different oxides of nitrogen, including but not limited to nitric oxide (NO), nitrogen dioxide (NO ₂ ), nitrogen peroxide (N ₂ O ₄ ), nitrogen pentoxide (N ₂ O ₅ ) and mixtures thereof. Furthermore, the term "lower valence nitrogen oxides" refers to nitrogen oxides that can still be oxidized to higher valence oxides. Nitrogen monoxide (NO) is the most preferred nitrogen oxide to be oxidized since up to 90% by weight of the nitrogen oxide in the exhaust of a typical FCC unit is NO. Thus, in a specific embodiment, the method relates to the reduction and control of NO.

The terms "exhaust", "moisture", "combustion exhaust stream", "combustion exhaust exhaust stream", "exhaust gas", "tail gas" and "exhaust gas stream" are sometimes used interchangeably herein. Likewise, the terms "wet gas scrubber", "scrubbing unit" and "scrubber" are sometimes used interchangeably herein.

The present invention provides a method for the efficient removal of _NOx species from exhaust gas streams. Because higher valence nitrogen oxides such as NO ₂ and N ₂ O ₅ are more soluble in water than lower valence nitrogen oxides, and are easier to remove from the system in the form of nitrate or nitrite , therefore, the oxidation of _NOx species to higher valence oxides is an effective method for removing _NOx species from exhaust gas streams. Thus, the method includes, at a time effective to oxidize at least a portion of the lower valence nitrogen oxides, particularly NO, contained in the exhaust stream to higher valence nitrogen oxides (e.g., NO ₂ and Under conditions of higher valence state), an effective amount of chlorine dioxide is added to the waste gas stream. These higher valence oxides can then be removed by alkaline solution absorption, reducing solution absorption, washing, ammonia injection, catalytic conversion, and water absorption. As used herein, an effective amount of chlorine dioxide is an amount capable of oxidizing at least a portion of oxidizable _NOx species present in the exhaust stream. By at least "a portion" we mean at least 20% to 80% by volume, preferably 40% to 90% by volume, more preferably 50% to 99% by volume, most preferably substantially all of the The lower valence oxide _NOx species are oxidized to higher valence nitrogen oxides.

The addition of sodium chlorite to the scrubber solution is described in US Patent No. 6,294,139. Typically, however, the reagent that oxidizes lower valence nitrogen oxides to higher valence nitrogen oxides ( _e.g. , NO to _NO2 and/or _N2O5 ) is actually chlorine dioxide rather than Not sodium chlorite. Therefore, sodium chlorite is usually injected into the exhaust gas stream together with an acidic component capable of disproportionating sodium ions and chlorine dioxide.

After disproportionation from sodium chlorite molecules, chlorine dioxide can also oxidize SO _x species to higher valence sulfur oxides. Such non-preferential oxidation reactions result in the need to inject relatively large amounts of sodium chlorite into the exhaust stream in order to reduce the _NOx species present in the exhaust stream to a satisfactory level. These larger quantities of sodium chlorite can adversely affect treatment plant hardware, cause problems with wastewater treatment, and increase the overall cost of reagents.

However, when used in the present process, chlorine dioxide is mixed with the exhaust stream at a point after at least a portion of the _SOx species present in the exhaust stream has been removed. The _SOx removal method used is not the gist of the present invention, and any effective method may be used. In a specific embodiment, _SOx removal is preferably employed to reduce the amount of _SOx species in the exhaust stream to below 100 ppm, preferably below 50 ppm, more preferably below 10 ppm prior to mixing sodium chlorite with the exhaust stream the following. It is most preferred to remove substantially all of the _SOx present in the exhaust stream prior to mixing the sodium chlorite with the exhaust stream. Non-limiting examples of _SOx removal methods suitable for use herein include wet desulfurization methods such as water washing, alkali washing, magnesium oxide washing, and ammonium washing, and dry desulfurization methods using, for example, manganese oxide or activated carbon. In a specific embodiment, _SOx species are removed by wet desulfurization, preferably by using a wet gas scrubber.

Chlorine dioxide can be used in slightly higher than stoichiometric amounts by mixing it with the exhaust stream after at least a portion of the _SOx species have been removed. In general, since the method by which chlorine dioxide converts lower valence nitrogen oxides to higher valence nitrogen oxides is complicated, so is the calculation of the stoichiometric amount. However, it is believed that the oxidation reaction of _NOx by chlorine dioxide can be represented by the following equation (without wishing to be bound by any theory or model, of course):

Equation 1:

In one embodiment, the amount of chlorine dioxide used is in the range of 3 to 8 moles of _ClO2 to 5 moles of NO, and in another embodiment 4 to 7 moles of _ClO2 to 5 moles of NO. In another embodiment, it is preferred to use a slightly higher than stoichiometric amount of sodium chlorite, eg, 3 to 4 moles of _ClO2 to 5 moles of _NOx .

It should be noted that in some cases the caustic contained in the scrubber will neutralize a portion of the HCl in Equation 1. In the case where the pH of the system is alkaline, the general oxidation reaction for the oxidation of lower valence nitrogen oxides to higher valence nitrogen oxides with chlorine dioxide is believed to be represented by the following equation ( Again without wishing to be bound by theory):

Equation 2:

Thus, in an alkaline environment, the amount of chlorine dioxide used is 3 to 8 moles of _ClO2 to 4 moles of NO, or 4 to 7 moles of _ClO2 to 4 moles of NO. In another embodiment, it is preferred to use a slightly higher than stoichiometric amount of sodium chlorite, eg, 3 to 4 moles of _ClO2 to 4 moles of _NOx .

After oxidizing at least a portion of the lower valence nitrogen oxides to higher valence nitrogen oxides, at least a portion of the higher valence nitrogen oxides is removed from the exhaust stream. In a specific embodiment, oxidation is followed by removal of 20% to 100% by volume of oxides of higher valence states of nitrogen, preferably 40% by volume to 80% by volume, more preferably 60% by volume to 90% by volume of oxides of higher valence states of nitrogen are removed after oxidation.

At least a portion of the higher valence nitrogen oxides may be removed by any effective method, except sodium chlorite absorption. Effective methods include, but are not limited to, the use of alkaline solutions such as aqueous caustic soda or reducing solutions such as aqueous sodium thiosulfate, catalytic conversion, and ammonia and hydrogen injection, as described in US Patent No. 3,900,554.

In another embodiment, the oxidized _NOx species are removed with water. JB Joshi, VV Mahajani and VA Juvekar in "Invited Review: Absorption of NO _x Gases", Chemical Engineering Communication, Vol. 33pp 1-92 describe the solubility of higher valence state oxides (such as SO _x and N ₂ O ₅ ) in water , which is hereby incorporated by reference. The most preferred embodiment of the method involves absorbing the oxidized _NOx compounds with water.

As noted above, it is preferred that at least a portion of the _SOx species be removed from the exhaust stream, preferably by means of moisture scrubbing. The effects of wet gas scrubbing include removal of attrition catalyst particulates and _SOx species. Thus, in a particular embodiment, the exhaust gas stream is directly contacted with chlorine dioxide somewhere downstream of the wet gas scrubber. Chlorine dioxide is able to oxidize more oxidizable _NOx species by contacting the exhaust stream downstream of wet gas scrubbing due to the lower amount of _SOx species in the gas stream competing with the reaction to oxidize _NOx . In addition, the relatively slow addition of chlorine dioxide required to oxidize lower valence _NOx species to higher valence nitrogen oxides is beneficial in overcoming at least some of the aforementioned problems, such as corrosion of equipment and wastewater treatment issues.

In another embodiment, the chlorine dioxide is mixed with the waste gas in a separation drum connected to a wet gas scrubber. The separation drum usually has components such as nozzles. In this embodiment, chlorine dioxide is sprayed through nozzles such that the contaminated waste gas stream contacts the chlorine dioxide as it is fed into the separation drum. Chlorine dioxide can first be mixed with water, preferably deionized water. The water acts as a carrier fluid to better disperse the chlorine dioxide. Also, in such embodiments, additional amounts of deionized water may be sprayed through the nozzles. The additional amount of deionized water refers to the amount of deionized water sufficient to absorb at least a portion of the higher valence nitrogen oxides.

In another embodiment, after the _SOx removal step, a greater amount of chlorine dioxide necessary to oxidize a given amount of _NOx species in the exhaust stream is mixed with the exhaust stream. The additional amount of chlorine dioxide enables the refiner to oxidize _SOx species remaining in the exhaust stream after the _SOx removal step to higher valence oxides. These higher valence SOx _- type oxides can then be removed by any effective method.

In another embodiment, the exhaust stream is subjected to an initial _NOx removal step to remove a portion of the _NOx , thereby reducing the amount of chlorine dioxide required to oxidize oxidizable _NOx remaining in the exhaust stream. In this initial _NOx removal step, at least 10% by volume, preferably 10% to 30% by volume, more preferably 20% to 60% by volume, most preferably 30% to 90% by volume, is removed before the exhaust gas stream is mixed with chlorine dioxide. % by volume of _NOx species originally present in the exhaust gas stream. The method of removing _NOx prior to mixing the exhaust stream with chlorine dioxide is not critical to the invention and may be any effective method.

The above description is directed to a preferred method for carrying out the invention. Those skilled in the art will recognize that other equally effective methods can be devised to carry out the effects of the present invention.

The following examples illustrate the effectiveness of the method of the present invention, but are not intended to limit the invention.

Example

The oxidation effect of chlorine dioxide on the oxidation of lower valence _NOx species oxides to higher valence state _NOx species oxides was tested in a bubble column. Chlorine dioxide was mixed with simulated scrubber fluid containing 1002 ppm _NOx . In this experiment, simulated scrubber liquor was flowed into a bubble column at a rate of 2 L/min, where it was mixed with 1.5 dm3 of a water/chlorine dioxide oxidation solution containing 107 ppm _ClO2 . The temperature of the bubble column was monitored during the experiment with a thermocouple device, and a temperature of 18°C was observed.

_NOx comparisons were performed on the bubble column by measuring the concentration of nitrogen oxides in the simulated scrubber liquor before and after mixing with the oxidizing solution. The comparison results are shown in Table 1.

Table 1 source compound Initial concentration (molar) Final concentration (molar) Percentage (%) of the total final concentration in the total initial concentration gas phase NO 0.0017 - gas phase NO ₂ 0.0004 0.0002 water box NO ₂ ^- - - water box NO ₃ ^- - 0.0019 total 0.0021 0.0021 100%

Claims (12)

1, a kind of be used for reducing both contained NO _xContain SO again _xNO in the waste gas stream of class material _xThe method of concentration, this method comprises:

A) from described waste gas stream, remove at least a portion SO _xThe class material makes SO thus _xFlowed by the waste gas of dilution;

B) can be with the oxidable NO of at least 20 volume % _xThe class material is oxidized under the condition of nitrogen oxide of higher valence state, makes described SO _xContacted with the chlorine dioxide of effective dose by the waste gas of dilution stream; And

C) by being selected from the method for the group of forming by alkaline solution absorption, reducing solution absorption, washing, water absorption, ammonia injection and catalyzed conversion, from the waste gas stream of handling, remove to the nitrogen oxide of the described higher valence state of small part.

2, according to the process of claim 1 wherein that described waste gas stream is from the fluid catalytic cracking treatment facility.

3, according to the method for claim 2, wherein realize described step a) by the wet type desulfurizing method.

4,, wherein use the reagent that is selected from manganese oxide and active carbon to realize described step a) by the dry type desulfurizing method according to the method for claim 2.

5, according to the method for claim 3, wherein the wet type desulfurizing method comprises washing, alkali cleaning, magnesia washing and ammonium washing.6, according to the method for claim 3, wherein remove described SO by the wet gas scrubbing method _xThe class material.

7, according to the method for claim 6, wherein said chlorine dioxide contacts in the downstream of the mist extractor of burner with described waste gas stream.

8, according to the method for claim 6, wherein said chlorine dioxide and described waste gas stream with knock-out drum that the wet gas scrubbing device links to each other in contact.

9, according to the method for claim 6, wherein said chlorine dioxide by at least one nozzle and described waste gas stream with knock-out drum that the wet gas scrubbing device links to each other in contact.

10, according to the process of claim 1 wherein described chlorine dioxide with mix with water before described waste gas stream contacts.

11, according to the process of claim 1 wherein the nitrogen oxide of removing at least a portion higher valence state by the ammonia injection method, in the ammonia injection method, ammonia is to inject with the form of the mixture of hydrogen.

12, according to the method for claim 10, wherein deionized water is injected with described chlorine dioxide.

13,, wherein before aforesaid right requires 1 described step a), remove at least a portion and be present in NO in the described waste gas stream at first according to the method for claim 3 _xThe class material.